An Essay...

Abstract.

Eros has been found to resemble Ida in almost all respects (e.g. saturated cratering,
S(IV) spectral type, bulk density, and probable ordinary chondritic composition) except for
one: it lacks the prominent color differences seen on Galileo's multispectral images of Ida.
The latter have been ascribed to recent, fresh units (impact penetration to bedrock and
irregular distribution of fresh ejecta around Ida) which contrast with the redder coloration
of most of Ida's surface, explained by an as-yet-unidentified "space weathering" process
that reddens Ida's surface with time. Eros exhibits the same reddened coloration as Ida,
presumably due to space-weathering inasmuch as NEAR's X-ray compositional
measurements indicate that Eros has an L- or LL-chondrite-like composition which should
not appear to be so red. Here we attribute the lack of any fresh units having ordinary
chondrite-like colors to a plausible 50 Myr hiatus in cratering of Eros following its
dynamical removal from the impact environment of the main asteroid belt, during which
space weathering has reached maturity everywhere on the body. Other possible
implications of such a cratering hiatus are also discussed, including implications for the
origin of the numerous boulders on Eros and the possible role of usually minor geological
processes in shaping the geomorphology of Eros.

This chart shows the position of Eros (and NEAR!) in the sky during the middle of
February 2000, about an hour before sunrise (when the sky is beginning to brighten), as
seen from a mid-northerly latitude. At this time, Eros will be low (about 13 degrees up)
and very faint (magnitude 14.2, requiring a sizeable backyard telescope). Eros is located
between two famous constellations of the zodiac, Scorpius and Sagittarius, low in the
southeastern sky in the middle of the Milky Way. The chart also shows the positions of
two other, much brighter, asteroids near Sagittarius (Juno and Vesta) as well as brilliant
Venus.

NEAR Arrives at Eros!

Crater Frequencies for Eros

Early Results: April 11, 2000

Impact crater populations on asteroids visited by spacecraft are
compared on these plots. The higher
locations on this plot indicate more craters; sparse cratering is lower on the graph.
Craters can never be
as numerous as indicated by the "geometric saturation" line, where they would lie
cheek-by-jowl, totally
crowding the surface. An important difference between asteroids is whether or not
large craters
dominate coverage of the surface compared with small craters (indicated by a line
sloping to the upper
right) or, instead, if smaller craters dominate the surface (a line sloping to the
upper left).

As shown by these plots, Eros has similar numbers of smaller craters as Mathilde
and
Ida (observed by
NEAR and Galileo, respectively). But Eros lacks the numerous huge craters
present on Mathilde. Eros
shares with Ida and Mathilde a trait for which Gaspra, so far, remains the
exception: their larger
numbers of craters indicate that they are rather old compared with the more
sparsely cratered Gaspra.

The top plot, prepared in late March, shows counts of a typical cratered region (for craters
several hundred meters in size) plus all northern-hemisphere craters of roughly 1 km
diameter and larger. The lower plot is based on earlier data from approach images of
Eros. More detailed discussion of the crater counts that went into the latter summary plot
is
provided below.

As the NEAR spacecraft closed in on Eros, images were taken that for the first time have
better spatial resolution than during the December 1998 flyby. Shown below are log-log
plots of crater diameter frequencies for all visible sides of Eros during a rotation on Day-
of-Year (DOY) 040 (circles); these plots are based on 11 craters larger than 1.5 km in
diameter. It is assumed, very crudely, that the counting area is 800 sq. km.

The next day (DOY 041), even higher resolution images were
available. 42 craters are analyzed and shown in solid squares. Counts at the smallest sizes
of both sets of data may be incomplete. It is beginning to look as though there is a fairly
steep "slope" to the power-law-like size-frequency relationship at crater sizes smaller than a
couple km. I have analyzed these data from deblurred images using computer routines
developed by Peter Thomas and Jonathan Joseph, from Cornell University.

NEAR's Encounter with Mathilde

On June 27, 1997, the NEAR spacecraft flew within less than 1200 km of the
nominally 60 km diameter, black, extremely-slowly spinning main-belt asteroid 253
Mathilde. The two chief scientific measurements obtained were a series of several
hundred images taken through several filters with the multi-spectral imager and a
radio science determination of the asteroid's mass.

Clark Chapman and Bill Merline, of SwRI, spent encounter week at the
Applied Physics Lab (APL) of Johns Hopkins University in Laurel, Maryland,
planning for the encounter and reducing the data obtained over the weekend
following the encounter, in preparation for the June 30th press conference. On
Encounter Day, Chapman participated in a panel discussing the encounter and early
results that was widely reported by the wire services.

The first results were (1) that Mathilde's shape is oddly carved by giant craters
about equal in diameter to the asteroid's radius and (2) that the mass of Mathilde is
only about 1/3rd that expected for an object of density 2.6 g/cc with a radius of 30 km.
Although we now think Mathilde is a little smaller than previously estimated, its
density is still surprisingly low. It
can be explained by a combination of inherently low density
carbonaceous material and a rubblized internal structure with voids, perhaps
augmented by the
gouges removed from the asteroid by the huge craters.